Modern capital jurisprudence relies on judicial instructions to guide the discretion of the capital jury in reaching a penalty-phase decision. Yet, previous social science research has raised doubts about the extent to which judicial instructions are generally understood by jurors and questioned their utility in producing intended effects. Subjects in this study were 491 upper-division undergraduate students (58 percent women and 42 percent men), who ranged in age from 19 to 36. After hearing a general description of the capital trial process in California, the subjects were read the standard California judicial instructions used in all capital cases tried within the State. Once the entire penalty instruction had been read aloud three times, the subjects were asked to define in writing the terms "aggravation" and "mitigation" as they were used in the context of the instruction. Subjects were then asked whether each factor was intended, in the context of the instruction, as aggravating or mitigating. Each of the specific factors was again read in sequence and then repeated. As each factor was read aloud for the final time, subjects indicated whether the presence of that particular factor in a capital case should be considered as either aggravating or mitigating. Researchers then analyzed the content of the subjects' written general definitions of aggravating or mitigating along a number of dimensions. They also tabulated their judgments about the aggravating and mitigating quality of each of the factors. Data show a widespread inability of the subjects to define accurately the central concepts of aggravation and mitigation in use in virtually every State that currently has a death penalty statute, as well as the inability to distinguish the sentencing significance of the enumerated factors juror are directed to use in reaching their life and death verdicts. Additionally, an inordinate focus on the circumstances of the crime, to the exclusion of other potentially important factors, was identified, as well as special problems in understanding the concept of mitigation in constitutionally required ways. 1 figure, 70 references, and appended California jury instructions in death-penalty cases